A routing protocol defines a process by which network devices, referred to as routers in packet-switched networks, communicate with each other to disseminate information that allows the routers to select routes between any two nodes on a computer network. One type of routing protocol, referred to as a link state protocol, allows routers to exchange and accumulate link state information, i.e., information describing the various links within the network. With a typical the link state routing protocol, the routers exchange information related to available interfaces, metrics and other variables associated with network links. This allows a router to construct its own topology or map of the network. One example of a link state protocol is Open Shortest Path First (OSPF).
Specifically, through application of the link state protocol, the routers exchange link information with other adjacent routers via Link State Advertisements (LSAs). A router generating an LSA typically floods the LSA throughout the network such that every other router receives the LSA. In this way, the receiving routers may construct and maintain their own network topologies using the link information exchanged via the LSAs.
One type of LSA that may be produced, referred to as “router LSA,” carries information that specifies the router that generated the LSA, the routers to which the generating router is directly coupled, and a sequence number. The information within the LSA specifying the generating router as well as the routers to which the generating router is coupled defines the links through the network. The sequence number defines the temporal status of the LSA. That is, the routers receiving the router LSA maintain previous LSAs from every other router and may compare the previous sequence number to the current sequence number to determine if the current LSA represents a more recent update than the previous LSA. If so, the receiving routers update their network topologies to reflect links specified in the current LSA. In this manner, each router may propagate link-state changes in the network topology quickly and efficiently to all other routers in the network.
In networks comprising a broadcast sub-network, such as an Ethernet sub-network, the routers within the broadcast sub-network may designate one of these broadcast routers to serve as a main forwarding conduit for all of the other broadcast routers' LSAs to the rest of the network. In addition to appearing as a network router within the network topology, this router, referred to as the designated router, appears as an additional logical or virtual router having links to all of the other routers. The selection and use of one of the routers as a designated router that appears as a central hub may greatly simplify the quantity of LSAs that need be transmitted as well as simplifying the network map or topology by eliminating unnecessary paths specifying the fully-meshed nature of the broadcast sub-network.
Unlike the other link-state routers within the network, the designated router generates a variation of the above described router LSA, i.e., a different type of LSA known as a “network LSA.” The network LSA designates link information in a manner similar to the router LSA, but further designates the subnet address of the broadcast network to identify the specific broadcast network to which the LSA pertains. All of the other broadcast routers within the subnet transmit their router LSAs to the designated router.
However, in some situations the specification of the subnet address within the network LSA may be problematic. For example, in networks having multiple broadcast sub-networks, the link state routing protocol may propagate a network topology that does not accurately represent the underlying physical network. The inaccurate network topology results, for example, when two or more broadcast sub-networks are identified by the same subnet address. As another example, inaccuracies may arise with the use of a link-state routing protocol in a network when one or more broadcast sub-networks are configured with a so called “/32” host address (i.e., a X.X.X.X/32 subnet address). In these instances of overlapping subnet address, the designated routers for each broadcast sub-network may possibly update each other's sub-network by issuing respective network LSAs bearing the same subnet address. Unable to distinguish between the sub-networks due to the overlapping subnet addresses, link-state routers within the different subnets that receive the successive LSAs will therefore maintain only one sub-network in their network topologies instead of the two actual sub-networks, thereby inaccurately representing the underlying physical network. This may result in forwarding loops or other network errors.